The present invention relates to a coated film. More particularly, it relates to a stretched polyester film having novel coating layer and showing excellent transparency, gloss, anti-block properties and adhesiveness.
Since stretched polyester films have excellent properties such as mechanical strength, dimensional stability, flatness, smoothness, heat resistance, chemical resistance and transparency, they have been widely used as a base film of magnetic recording medium, a film for printing, magnetic card, synthetic paper, etc.
While the polyester films have such excellent properties, they are poor in adhesiveness, which is a problem common to the plastic films in general. For instance, the polyester films have poor adhesiveness to printing inks (printing ink for cellophane, chlorinated PP ink, ultraviolet curing ink, magnetic ink, etc.), thermal transfer ink, magnetic coatings, adhesives (laminating adhesives, wood adhesives, etc.), topcoating materials (releaser, ink image receiving layer, gelatin, polyvinyl alcohol, polyvinyl acetal, cellulose acetate, cellulose butyroacetate, methyl cellulose, carboxymethyl cellulose, etc.), and deposited metals and inorganic materials (aluminum, silver, gold, ITO, silicon oxide, aluminum oxide, etc.).
As a method of solving the above problem, it is known to provide a coating layer on the polyester film surface. Especially the method in which coating is conducted in the course of the film forming process is preferable in view of economic and characteristics thereof. This technique is called xe2x80x9cin-line coating.xe2x80x9d In a typical instance of this method, coating is conducted after longitudinal stretching and before transverse stretching, and then transverse stretching and heat setting are conducted. Various types of compounds including polyesters, poly(meth)acrylates, polyurethanes, polyvinylidene chloride, polyolefins and silane coupling agents can be used as the coating compounds. Coating with these compounds provides, in many cases, improvement of adhesiveness in some way or other, but on the other hand, it is causative of the phenomenon of the films sticking to each other (so-called xe2x80x9cblockingxe2x80x9d). In order to prevent such blocking and to improve adhesiveness, a crosslinking agent is further added in many cases.
Generally, the coating layer is hardened when a crosslinking agent is added. So, there arises a problem of a stretching conformability of the coating layer especially when the film is stretched after coating such as mentioned above. More specifically, if the coating layer is poor in stretchability, it fails to be stretched uniformly and may be fractured finely in the polyester film stretching step, thus forming microcracks in the coating layer in many cases. Such cracks in the coating layer cause such defects as reduction of adhesiveness to the topcoats, etc., and clouding of the coating layer due to scattering of light by fine unevenness of the coating layer surface. Thus, the problem arises that even when the base polyester film is not transparent, glossiness of the film surface lowers.
Therefore, in production of the coated and stretched polyester films, there is required incorporation of a crosslinking agent which is capable of satisfying the above-said antagonistic requirements simultaneously. As this type of crosslinking agent, the oxazoline-based crosslinking agents (for instance, those disclosed in Japanese Patent Publication (KOKOKU) No. 8-2979) have generally been known, but the desired properties can not be obtained when these crosslinking agents are simply applied to the method in which coating is performed in the polyester film forming process.
As a result of the present inventors"" earnest studies to solve the above problem, it has been found that by forming a coating layer on a polyester film by applying on the film a coating solution containing a copolymer comprising (meth)acrylonitrile, styrene and an oxazoline-containing monomer, the above problem can be solved.
The present invention has been attained on the basis of the above finding.
The object of the present invention is to provide a polyester film which has excellent transparency and glossiness of the coating layer and also is excellent in adhesiveness and anti-block properties.
To attain the above aim, in the first aspect of the present invention, there is provided a coating film comprising:
a polyester film and
at least one coating layer formed by coating a coating solution containing a polymer (A) comprising a copolymer of oxazoline group-containing monomer, (meth)acrylonitrile and styrene, having an equivalent weight of oxazoline group of less than 300 g/equivalent, and drying and stretching thereof.
The present invention will be described in detail below.
The polyester constituting the polyester film of the present invention is the one obtained by using an aromatic dicarboxylic acid or an ester thereof and a glycol as the main starting materials. Examples of the preferable polyester have not less than 70% of the recurring structural units of ethylene terephthalate units, ethylene-2,6-naphthalate units, 1,4-cyclohexane terephthalate units or ethylene isophthalate units, and are especially preferably polyethylene terephthalate and polyethylene-2,6-naphthalate. It may contain other components as far as the above condition is met.
As the aromatic dicarboxylic acid, it is possible to use, for example, terephthalic acid, 2,6-naphthalenedicarboxylic acid, isophthalic acid, phthalic acid, adipic acid, sebasic acid, oxycarboxylic acids (such as p-oxyethoxybenzoic acid) and the like either singly or as a mixture of two or more of them. As the glycol, ethylene glycol, diethylene glycol, propylene glycol, butanediol, 1,4-cyclohexanedimethanol, neopentyl glycol, polyalkylene glycol and the like can be used either singly or as a mixture of two or more of them.
The intrinsic viscosity of these polyesters is usually not less than 0.45, preferably in the range of 0.50 to 1.0, more preferably 0.52 to 0.80. When the intrinsic viscosity is less than 0.45, there may arise the problems such as reduced productivity in manufacture of the film and reduced mechanical strength of the produced film. On the other hand, it is preferable that the intrinsic viscosity does not exceed 1.0 in view of melt extrusion stability of the polymer.
In the polyester film of the present invention, it is possible to contain the particles in the polyester so as to form appropriate projections on the film surface to provide proper slip characteristics to the film to thereby improving its handling qualities. Examples of such particles usable in the present invention include the inorganic particles such as the particles of calcium carbonate, calcium phosphate, silica, kaolin, talc, titanium dioxide, alumina, barium sulfate, calcium fluoride, lithium fluoride, zeolite, molybdenum sulfide, etc., the organic particles such as the particles of crosslinked polymers, calcium oxalate, etc., and the precipitated particles produced in polyester polymerization.
The size and amount of the particles to be contained in the film in the present invention, although variable depending on the purpose of use of the film, are generally as described below. The average particle size is preferably in the range of 0.005 to 5.0 xcexcm, more preferably 0.01 to 3.0 xcexcm. When the average particle size exceeds 5.0 xcexcm, the film surface is roughened to an excess degree. Also, in a thin film, the insulating performance may deteriorate. Further, the particles become liable to fall off the film surface, causing so-called xe2x80x9cdrop-off of particlesxe2x80x9d in use of the film. When the average particle size is less than 0.005 xcexcm, the effect of improving the slip characteristics of the film tends to lower because of insufficient formation of the projections by the particles. That is, the effect of improving the slip characteristics may not be provided unless the particles are contained in large quantities, but on the other hand, mechanical properties of the film may be impaired if the particles are added in large quantities.
The content of the particles is preferably not less than 30% by weight, more preferably 0.010 to 20.0% by weight, based on the weight of polyester. When the particle content exceed the above-defined range, mechanical properties of the film may be deteriorated. The lower threshold content varies depending on the purpose of use of the film. The smaller the particle content, the better for a high-transparency film. A smaller particle content is also preferable for providing appropriate slip characteristics. In use for magnetic recording, slip characteristics of the film is an important quality factor, and usually a particle content of at least 0.1% by weight is necessary although it is variable depending on the size of the particles to be added. In the case of a white film produced by adding a white pigment such as calcium carbonate or titanium oxide, a particle content of not less than 2% by weight is usually needed. This is required in the case of producing a film with a high light-shielding effect; the lower threshold value of the particle content may be smaller in the case of a half-transparent film.
It is possible to contain in the film two or more types of these particles or the particles of the same type but differing in size. In either case, it is preferable that the average size of the particles contained in the film and their total content fall in the above-defined ranges.
In producing the particle-containing polyester, the particles may be added either in the course of the polyester synthesis or directly to the polyester. In case where the particles are added in the course of the polyester synthesis, it is preferable to use a method in which a slurry is formed by dispersing the particles in ethylene glycol or the like, and this slurry is added in a preferable phase of the polyester synthesis. On the other hand, in case where the particles are added directly to the polyester, a method is preferably used in which the dried particles or a slurry thereof formed by dispersing the particles in water or an organic solvent having a boiling point of not more than 200xc2x0 C. is added and mixed with the polyester by using a double-screw extruder. If necessary, the particles to be added may have been subjected to such treatment(s) as pulverization, dispersion, classification and filtration before addition.
In order to adjust the particle content, a method in which first a master material containing the particles in a high concentration is prepared in the manner described above, and in the film forming process, this master preparation is diluted with a material which is substantially free of the particles to thereby adjust the particle content, is preferably used.
As the additives other than the said projection-forming agent, there can be contained as required such materials as antistatic agent, stabilizer, lubricant, crosslinking agent, anti-block agent, antioxidant, colorant (dye and pigment), light screen, ultraviolet absorber, etc. Thus, the polyester film according to the present invention may be, for instance, a colored film or a foamed thin sheet containing a plurality of fine air cells.
The polyester film of the present invention may be of a multi-layer structure as far as the finally satisfied properties of the film satisfy the requirements of the present invention. For instance, it may be a co-extruded laminated film. In this case, the above description relating to the base film applies to the polyester of the outermost surface layer. The films of the other (inner) layers may be made of any type of polyester, plastic material, paper or cloth. Such inner layer films may be, for instance, foamed films containing a plurality of fine air cells.
The polyester film of this invention may be either a monoaxially stretched film or a biaxially stretched film as far as it is a stretched polyester film. However, the biaxially stretched film is more widely used industrially.
Biaxial stretching of the polyester film is preferred by either simultaneous biaxial stretching or successive biaxial stretching, especially successive biaxial stretching is usually used. According to the successive biaxial stretching operation, the melt extruded polyester is cooled on a cooling drum to form a non-stretched film, and the non-stretched film is first stretched in the machine direction (longitudinal stretching) by a set of rolls differing in peripheral speed from each other, and then further stretched while held by a clip in the direction orthogonal to the longitudinal stretching direction. As a modification of this operation, both longitudinal stretching and transverse stretching may be conducted in several stages. Or they may be conducted alternatively until the film is stretched to the desired degree. This technique is used where a high-strength film is produced by a re-stretching method.
Now, the polymer (A) used for the coating composition in the present invention is explained.
The polymers of the oxazoline compounds according to the present invention can be synthesized by using a monomer containing an oxazoline compound as at least one of the starting monomers. As the oxazoline compound, there can be used the 2-oxazoline, 3-oxazoline and 4-oxazoline compounds. Especially the 2-oxazoline compounds are preferable because of highly reactive performance and industrial availability.
Examples of the monomers having oxazoline groups include, but are not limited to, 2-vinyl-2-oxazoline (VOZO), 5-methyl-2-vinyl-2-oxazoline (MVOZO), 4,4-dimethyl-2-vinyl-2-oxazoline (DMVOZO), 4,4-dimethyl-2-vinyl-5,6-dihydro-4H-1,3-oxazine (DMVOZI), 4,4,6-trimethyl-2-vinyl-5,6-dihydro-4H-1,3-oxazine (TMVOZI), 2-isopropenyl-2-oxazoline (IPOZO), 4,4-dimethyl-2-isopropenyl-2-oxazoline (DMIPOZO), 4-acryloyl-oxymethyl-2,4-dimethyl-2-oxazoline (AOZO), 4-methacryloyl-oxymethyl-2,4-dimethyl-2-oxazoline (MAOZO), 4-methacryloyl-oxymethyl-2-phenyl-4-methyl-2-oxazoline (MAPOZO), 2-(4-vinylphenyl)-4,4-dimethyl-2-oxazoline (VPMOZO), 4-ethyl-4-hydroxymethyl-2-isopropenyl-2-oxazoline (EHMIPOZO), and 4-ethyl-4-carboethoxymethyl-2-isopropenyl-2-oxazoline (EEMIPOZO).
Vinyloxazolines easily undergo radical polymerization in the presence of AIBN or BPO to produce a polymer having oxazoline rings in the side chain. Vinyloxazolines also produce similar poly(vinyloxazolines) through anionic polymerization using n-butyl lithium as catalyst. There are the methods not resorting to the use of the monomers having oxazoline rings, for example, a method comprising isomerization reaction of poly(methacryloylazilidine), for the synthesis of poly(vinyloxazolines).
The polymers (A) having oxazoline rings used in the present invention is necessary copolymerized with (meth)acrylonitrile and styrene. Further, the polymer (A) may be copolymerized with other optional copolymerizable monomers. Other monomers may as well be copolymerized, but in either case it is essential that an oxazoline-containing monomer, (meth)acrylonitrile and styrene are copolymerized as the primary three monomeric constituents of the polymer (A). The xe2x80x9cprimary three monomeric constituentsxe2x80x9d mean xe2x80x9cthe three monomers first to third highest in contentxe2x80x9d in the monomers constituting the oxazoline-containing copolymers, and the existence of other copolymerized monomers may be present therein. Further, since the oxazoline group density needs to be high, the oxazoline equivalent is not more than 300 g/equivalent, preferably not more than 250 g/equivalent. On the other hand, the oxazoline equivalent is usually not less than 97, preferably more than 97, more preferably not less than 100. The contents of (meth)acrylonitrile and styrene units are more than 0% by weight based on the weight of polymer constituting the coating layer, respectively. On the other hand, the content of oxazoline-containing monomer unit is less than 100% by weight based on the weight of polymer constituting the coating layer.
It is also preferable that the polymers (A) are dispersible and soluble in water, especially that they are soluble in water. They may be made dispersible and/or soluble in water by using an organic solvent mixable with water. In this case, a coating film with excellent transparency and anti-block properties is formed even when stretching is conducted after coating.
The coating layer in the present invention may contain the compounds other than the said polymer (A) having oxazoline groups. Examples of the components other than the polymer (A) include binder, crosslinking agent, organic particles, inorganic particles, wax, surfactant, defoaming agent, coating properties improver, thickener, antioxidant, ultraviolet absorber, foaming agent, dye and pigment.
Especially, it is preferable for a good balance of properties to use a combination of a polyester, polymer(s) of vinyl monomer and the particles in addition to a polymer (A) having oxazoline groups. Use of such a combination of materials affords the advantages of minimized coloration in melt extrusion for reuse of the coated film, high adhesion to the base polyester film, good water resistant adhesion, and improved slip characteristics. If necessary, a wax may be additionally used.
As the components of the polyester used as a binder for the coating layer in the present invention, there can be exemplified the polyvalent carboxylic acids and polyvalent hydroxyl compounds such as listed below. Examples of the polyvalent carboxylic acids include terephthalic acid, isophthalic acid, orthophthalic acid, phthalic acid, 4,4xe2x80x2-diphenylcarboxylic acid, 2,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 2-potassiumsulfoterephthalic acid, 5-sodiumsulfoisophthalic acid, adipic acid, azelaic acid, sebasic acid, dodecanedicarboxylic acid, glutaric acid, succinic acid, trimellitic acid, trimesic acid, trimellitic acid anhydride, phthalic anhydride, p-hydroxybenzoic acid, trimellitic acid monopotassium salt, and ester-forming derivatives thereof. Examples of the polyvalent hydroxyl compounds include ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, 2-methyl-1,5-pentanediol, neopentyl gylcol, 1,4-cyclohexane dimethanol, p-xylylene glycol, bisphenol A-ethylene glycol adduct, diethylene glycol, triethylene glycol, polyethylene glycol, polypropylene glycol, polytetramethylene glycol, polytetramethylene oxide glycol, dimethylolpropionic acid, glycerin, trimethylolpropane, sodium dimethylolethylsulfonate, and potassium dimethylolpropionate. The polyesters can be synthesized by the ordinary polycondensation reaction.
Besides the above-mentioned, the composite polymers having such polyester components as so-called acrylic graft polyesters such as disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 1-165633 and polyester polyurethanes produced by chain-lengthening polyester polyols with an isocyanate are also included in the polyesters usable for coating composition binder in the present invention.
The polyester for coating composition binder used in the present invention is preferably the one which is capable of forming a coating composition using water as medium. The polyester may be of the type which is capable of forming a coating composition after forced dispersion by a surfactant or the like, but it is preferably a self-dispersing type polyester having hydrophilic nonionic components such as polyethers or cationic groups such as quaternary ammonium salts, more preferably a water-soluble or water-dispersible polyester resin having anionic groups. The xe2x80x9cpolyester having anionic groupsxe2x80x9d is a polyester to which a compound having anionic groups has been attached by means of copolymerization or grafting. The compound is properly selected from sulfonic acid, carboxylic acid, phosphoric acid and their lithium salt, sodium salt, potassium salt and ammonium salt.
The amount of the anionic groups in the polyester is preferably in the range of 0.05 to 8% by weight. When the amount of the anionic groups is less than 0.05% by weight, the produced polyester resin tends to deteriorate in water solubility or water dispersibility, while when the amount of the anionic groups exceeds 8% by weight, water resistance of the coating layer may deteriorate, or the obtained films may become liable to stick to each other (blocking) after moisture absorption.
The polymer of vinyl monomer used for the coating layer in the present invention are the polymers comprising the polymerizable monomers having carbon-carbon double bonds, specifically acrylic or methacrylic monomers. These polymers may be either homopolymers or copolymers. Copolymers of these and other polymers (such as polyesters and polyurethanes), which may be block or graft copolymers, are also included in the concept of said polymers in the present invention. They also include the polymers (polymer mixtures in some cases) obtained by polymerizing the polymerizable monomers having carbon-carbon double bonds in a polyester solution or a polyester dispersion; the polymers (polymer mixtures in some cases) obtained by polymerizing the polymerizable monomers having carbon-carbon double bonds in a polyurethane solution or a polyurethane dispersion; and the polymers (polymer mixtures in some cases) obtained by polymerizing the polymerizable monomers having carbon-carbon double bonds in other polymer solution or dispersion.
The polymerizable monomers having carbon-carbon double bonds usable in the present invention are not specified, but they include as typical examples thereof the following: monomers having various types of carboxyl groups, such as acrylic acid, methacrylic acid, crotonic acid, itaconic acid, fumaric acid, maleic acid, citraconic acid and their salts; monomers having various types of hydroxyl groups, such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, monobutylhydroxyl fumalate and monobutylhydroxyl itaconate; various (meth)acrylic esters such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate and lauryl (meth)acrylate; various types of nitrogen-containing vinyl monomers such as (meth)acrylamides, diacetone acrylamides, N-methylol acrylamides and (meth)acrylonitrile; styrene and various styrene derivatives such as a-methylstyrene, divinylbenzene and vinyltoluene, and various vinyl esters such as vinyl acetate and vinyl propionate; various silicon-containing polymerizable monomers such as xcex3-methacryloxypropyltrimethoxysilane, vinyltrimethoxysilane and Silaplane FM-07 (a methacryloyl silicon macro-monomer produced by Chisso Corp.); phosphorus-containing vinyl monomers; various types of halogenated vinyls such as vinyl chloride, vinylidene chloride, vinyl fluoride, trifluorochloroethylene, tetrafluoroethylene, chlorotrifluoroethylene and hexafluoropropylene; and various types of conjugated dienes such as butadiene.
The method of producing the polymers from the above-mentioned vinyl monomers is not specified in the present invention; the ordinary methods can be used for their production. For example, the polymers can be obtained by mixing the monomers such as mentioned above and a polymerization initiator with an organic solvent, and heating the mixture with stirring, or by adding dropwise the monomers and a polymerization initiator to an organic solvent while heating with stirring. Also, an organic solvent, the monomers and a polymerization initiator may be polymerized in an autoclave under high pressure. It is further possible to use emulsion polymerization or suspension polymerization using water instead of an organic solvent and if necessary additionally using a surfactant.
As the polyurethane used for the coating layer in the present invention, it is possible to use the known polyurethanes such as disclosed in Japanese Patent Publication (KOKOKU) Nos. 42-24194, 46-7720, 46-10193 and 49-37839, and Japanese Patent Application Laid-Open (KOKAI) Nos. 50-123197, 53-126058 and 54-138098, and their derivatives. These polyurethanes can be produced by reacting polyisocyanates, polyols and if necessary a chain-lengthening agent and/or a crosslinking agent.
Examples of the polyisocyanates usable for the reaction include tolylene diisocyanate, phenylene diisocyanate, 4,4xe2x80x2-diphenylmethane diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate, 4,4xe2x80x2-dicyclohexylmethane diisocyanate and isophorone diisocyanate.
As polyols, it is possible to use polyetherpolyols such as polyoxyethylene glycol, polyoxypropylene glycol and polyoxytetramethylene glycol; polyesterpolyols such as polyethylene adipate, polyethylene-butylene adipate and polycaprolactone; acrylic polyols, and castor oil. Usually, polyols having a molecular weight of 300 to 2,000 are used.
As the chain-lengthening agent or crosslinking agent, there can be used, for instance, ethylene glycol, propylene glycol, butanediol, diethylene glycol, trimethylolpropane, hydrazine, ethylenediamine, diethylenetriamine, 4,4xe2x80x2-diaminodiphenylmethane, 4,4xe2x80x2-diaminodicyclohexylmethane, and water.
The said polyurethanes preferably have an anionic substituent, for example, xe2x80x94SO3H group, xe2x80x94OSO3H group, xe2x80x94COOH group and their ammonium salts, alkaline metal salts and alkaline earth metal salts, for bettering solubility in the solvents using water as main medium. The following methods (1) to (3) can be cited as means for preparing these polyurethanes, but the present invention is not limited to these embodiments.
(1) A method using the compounds having an anionic substituent as the polyisocyanate, polyol and chain-lengthening agent.
For instance, a polyisocyanate having an anionic substituent can be obtained by sulfonating an aromatic isocyanate compound. It is also possible to use an isocyanate compound having a sulfuric ester salt or diaminocarboxylic acid salt of an amino-alcohol.
(2) A method comprising reacting a compound having an anionic substituent with the unreacted isocyanate group of the produced polyurethane.
As the compound having an anionic substituent, there can be used the compounds having, for example, a bisulfite, aminosulfonic acid or its salts, aminocarboxylic acid or its salts, sulfuric ester of an amino-alcohol or its salts, or hydroxyacetic acid or its salts as the anionic substituent.
(3) A method comprising reacting an active hydrogen-containing group (OH, COOH, etc.) of the polyurethane with a specific compound.
As the specific compound, there can be used, for example, dicarboxylic acid anhydrides, tetracarboxylic acid anhydrides, sultone, lactone, epoxycarboxylic acid, epoxysulfonic acid, 2,4-dioxo-oxazolidine, isatonic acid anhydrides, and phostone. It is also possible to use the 3- to 7-membered ring cyclic compounds having a salt type group or a group capable of forming a salt after ring opening, such as carbyl sulfate.
The coating composition used in the present invention is preferably the one using water as medium for reasons of hygienic safety, but the composition may contain an organic solvent provided that it does not overstep the concept of the present invention, and that the compound containing such a solvent can be dissolved in water.
The solids concentration of the coating composition used in the present invention is not specifically defined, but it is preferably in the range of 0.4 to 65% by weight, more preferably 1 to 30% by weight, most preferably 2 to 20% by weight.
As means for applying the said coating solution on a polyester film, it is possible to use the various coating devices such as illustrated in Yuji Harasaki: Handbook of Coating Devices and Their Operational Techniques, Sogo Gijutsu Center, 1990, which include forward roll coater, reverse roll coater, gravure coater, knife coater, blade coater, rod coater, air doctor coater, curtain coater, fountain coater, kiss-coater, kiss-roll coater, bead coater, dip coater, screen coater, cast coater, spray coater, and other types of coaters and coating systems such as impregnator and LB coating system.
The pre-coated film may be subjected to a chemical treatment or discharge treatment for improving the coating properties and adhesion of the coating layer to the film. Also, a discharge treatment may be applied on the coating layer after its formation for improving the coating properties and adhesion of the coating layer to the biaxially stretched polyester film of the present invention.
The thickness of the coating layer, determined as that of the dry solids in the final product, preferably falls in the range of 0.005 to 10.0 xcexcm, more preferably 0.01 to 2.0 xcexcm, especially preferably 0.015 to 0.2 xcexcm. The thickness of coating layer is preferably made as small as it can be within the above range. When the thickness of coating layer exceeds 10.0 xcexcm, there may arise problems of blocking, etc., become conspicuous. On the other hand, when the thickness of coating layer is less than 0.005 xcexcm, it may become unable to obtain the preferred performance of the film, and also there tends to take place unevenness or coating holes.
The coating step may be conducted at various stages in the polyester film producing process. For example, coating may be performed on the non-stretched film, the coated film being then stretched mono- or biaxially. Or coating may be conducted on the monoaxially stretched film, and this coated monoaxially stretched film may be offered as a product or may be further stretched to provide a coated biaxially stretched film. The features of the present invention are displayed to the maximum when coating is conducted in the polyester film producing process and the coated film is then stretched. It is especially preferable to carry out coating before tentering as this makes it possible to minimize or unnecessitate enlargement of the drying oven. In a typical embodiment of the process of the present invention, the said aqueous coating composition is applied on a film which has been stretched in the machine (longitudinal) direction, and this coated film is dried, stretched transversely, heat set and taken up. If necessary, the thus treated film may be re-stretched in the longitudinal direction and relaxed. Drying of the coating layer is preferably conducted during preheating before transverse stretching of the polyester film or in the course of transverse stretching. In the similar embodiments of the present invention, coating is conducted on the non-stretched film and this coated film is monoaxially stretched or biaxially stretched simultaneously by a tenter.
The coating layer in the present invention may be of a single-layer or multi-layer structure, or it may be provided as a single layer or plural layers in a multi-layer structure.
A typical film producing process according to the present invention will be described in further detail.
The starting polyester material is supplied to an extruder, melted at a temperature above the melting point of the polyester and extruded as a molten sheet from a slit die. The molten sheet is rapidly cooled to a temperature below the glass transition point and solidified on a rotary cooling drum to obtain a non-stretched sheet of a substantially amorphous state. In this operation, it is preferable to elevate adhesion between the sheet and the rotary cooling drum for improving flatness of the sheet, and for this purpose, an electrostatic pinning method and/or liquid coat adhering method are preferably used in the present invention.
The thus obtained non-stretched coated sheet is first stretched in the machine direction. This stretching is preferably conducted at a temperature in the range of 70 to 150xc2x0 C. for a stretch ratio of 2.5 to 6 times. Stretching may be conducted in a single step or in two or more steps. In the present invention, the said coating solution is applied on at least one side of the sheet and then dried at an arbitrary stage in the process. Most preferably, this operation is conducted at a stage after longitudinal stretching and before transverse stretching. Then the resulting monoaxially oriented film is once cooled to a temperature below the glass transition point or not cooled but preheated to a temperature range of, for example, 90 to 150xc2x0 C., and further stretched 2.5 to 5 times, preferably 3.0 to 4.5 times transversely, namely in the direction orthogonal to the direction of longitudinal stretching, to obtain a biaxially oriented film. If necessary, preheating may be reinforced. The thus obtained film is heat treated for a period of one second to 5 minutes with an elongation of not more than 30% or under a limited shrinkage or constant length. In this process, in order to attain an optimal degree of heat shrinkage in the longitudinal direction, it is possible to incorporate an appropriate technique, such as relaxing the film by an amount of not more than 10%, preferably not more than 5%, in the longitudinal direction in the step of heat treatment or after the heat treatment. The heat treatment temperature, though variable depending on the stretching conditions, is preferably in the range of 180 to 250xc2x0 C., more preferably 200 to 230xc2x0 C. When the heat treating temperature exceeds 250xc2x0 C., the film density becomes too high. Also, part of the coating layer may be thermally decomposed. On the other hand, when the heat treating temperature is below 180xc2x0 C., there results a too high rate of heat shrinkage of the film.
In the manufacture of the coated films according to the process of the present invention, defectives are produced at a certain rate. So, recycling of such defective products is of great industrial interest. If no reclaimed material is mixed in the starting polyester, the production cost elevates, but too much mixing of the reclaimed material causes tinting in the melt extrusion or other steps. It also impairs the dynamic properties of the produced polyester film. The reclaimed material is preferably mixed into the material for the base polyester film. Though variable depending on the polyester film thickness, coating layer thickness, polymer (A) content, production yield and other factors, the reclaimed material is preferably mixed in the base polyester film in such a manner that the amount of the coating layer in the reclaimed coating film is not more than 10% by weight based on the weight of the material for the base polyester film. Mixing of the amount of the coating layer in the reclaimed material in excess of 10% by weight may cause deterioration of the optical and dynamic properties such as mentioned above of the produced film.
The film according to the present invention is the one having excellent adhesiveness, transparency and anti-block properties. This film is therefore particularly preferred for uses where transparency is required. Further, the coating layer provided according to the present invention is of value for the translucent or opaque films, too. It may appear that transparency of the coating layer is not required for the translucent or opaque films, but this does not hold true for all the cases. Transparency of the coating layer is associated with its gloss; a clouded coating layer reduces its gloss. That is, it is of much account for all the polyester films to be able to afford adhesiveness to the base film while maintaining its gloss. A glossy coating layer has a smooth surface, making the films liable to stick to each other. The film according to the present invention has eliminated this problem.
The film of the present invention is a polyester film having a novel coating layer with high transparency and glossiness and also excellent in adhesiveness and anti-block properties.